In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for manufacturing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lower fuel consumption for vehicles using such tires.
In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known in the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent, such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like and a vulcanizing system such as sulfur-containing vulcanizing (curing) system.
Precipitated silica has been increasingly used as a reinforcing particulate filler in carbon black-filled elastomer compositions of tires and other mechanical goods. Silica-filled elastomer stocks, however, exhibit relatively poor processability.
The use of starch polymers in various industries has been known for many years. Starch polymers have been found to be particularly useful in the food industry and many non-food industries, such as mining, adhesive, paper-making and chemical production industries.
Starch is a carbohydrate polymer having the following repeating unit: 
The starch polymer is comprised of two distinct types of D-glucopyranose polymers, namely about 25 percent amylose, which is an essentially linear polymer of xe2x80x9c-D-glucopyranosyl units linked by 1xe2x86x924 glucosidic bonds, and about 75 percent amylopectin., which is a branched polymer comprising xe2x80x9c-D-glucopyranosyl units linked by 1xe2x86x924 glucosidic bonds and 1xe2x86x926 branch points. For a general discussion of starch polymers see Kirk-Othmer Concise Encyclopedia of Chemical Technology, Fourth Edition, John Wiley and Sons, Inc., Jacqueline I. Kroschwitz, Editor, pp.1882-1884 (1999).
The use of starch has been suggested as a reinforcing filler for various elastomer compositions, as a replacement for silica fillers. Elastomer compositions are conventionally compounded at a temperature range of about 140xc2x0 C. to about 170xc2x0 C., and the ultimate physical properties of a cured elastomer composition depend on the degree to which the elastomer additives are effectively or homogeneously dispersed within the elastomer matrix. Typically, the physical properties of a cured elastomer composition are improved as conventional elastomer additives become more homogeneously blended in the elastomer matrix. However, the softening temperature of the starch polymer is above 200xc2x0 C., and cannot soften or melt and be efficiently blended or dispersed in the elastomer matrix at conventional elastomer compounding temperatures. For this reason, the use of starch polymers as a reinforcing filler for elastomer compositions has been limited.
To overcome the limitations of the high softening point of the starch polymer, the prior art discloses a starch/plasticizer composite for reinforcing rubber compositions. The disclosed starch/plasticizer composite employs plasticizers, such as poly(ethylene vinyl alcohol) and/or cellulose acetate or any suitable plasticizer that results in a starch/plasticizer composite having a melting point below that of the starch alone, and that enables the starch to be more processable with conventional rubber or elastomer composition processing equipment. In order for the starch to be effectively dispersed in the elastomer matrix, the starch must first be blended with the poly(ethylene vinyl alcohol) and/or cellulose acetate to lower the softening point of the starch to between 110xc2x0 C. and 160xc2x0 C. to provide for the homogenous dispersion of the starch in the elastomer matrix.
There still remains a need in the art of elastomer compounding to develop cost-effective, renewable filler materials useful as a partial or total replacement for conventional filler materials, such as carbon black and silica, in elastomer compositions, and to improve the processability and physical properties of reinforced elastomer compositions.
The present invention is directed to a reinforced elastomer or rubber composition and to pneumatic tires having tread stocks produced with the reinforced elastomer composition. The invention is more particularly directed to a vulcanizable elastomer composition containing a starch/emulsion styrene butadiene copolymer reinforcing filler as a partial replacement for silica filler.
The present invention, provides a vulcanizable elastomer composition comprising 100 parts by weight of an elastomer; about 5 to about 80 parts by weight of reinforcing filler including a starch/emulsion styrene butadiene copolymer, per 100 parts of the elastomer; about 1 to about 6 parts by weight of a coupling agent, per 100 parts of the elastomer, said coupling agent having a moiety reactive with the hydroxyl groups of the starch/emulsion styrene butadiene copolymer and a second moiety reactive with said elastomer; about 40 to about 115 parts of an additional reinforcing filler, per 100 parts of the elastomer, wherein said filler is selected from the group consisting of carbon black, silica and mixtures thereof; and about 0.2 to about 5 parts of a curing agent, per 100 parts of the elastomer.
The present invention further provides a process for the preparation of a vulcanized elastomer composition comprising reacting 100 parts of an elastomer with about 10 to about 80 parts by weight of reinforcing filler, per 100 parts of the elastomer, the filler including a starch/emulsion styrene butadiene copolymer; about 1 to about 6 parts by weight of a coupling agent, per 100 parts of the elastomer, the coupling agent having a moiety reactive with the hydroxyl groups of the starch/emulsion styrene-butadiene copolymer and a second moiety reactive with the elastomer; about 40 to about 115 parts of an additional reinforcing filler, per 100 parts of the elastomer, wherein the additional filler is selected from the group consisting of carbon black, silica and mixtures thereof; and about 0.2 to about 5 parts of a curing agent, per 100 parts of the elastomer; and effecting vulcanization.
The present invention also provides a pneumatic tire having tread stock vulcanized from an elastomeric composition comprising 100 parts by weight of an elastomer; about 5 to about 80 parts by weight of reinforcing filler, per 100 parts of said elastomer, said filler including a starch/emulsion styrene butadiene copolymer; about 1 to about 6 parts by weight of an alkylalkoxysilane coupling agent, per 100 parts of said elastomer, said coupling agent having a moiety reactive with the hydroxyl groups of the starch/emulsion styrene butadiene copolymer and a second moiety reactive with said elastomer; about 40 to about 115 parts of an additional reinforcing filler, per 100 parts of said elastomer, wherein said additional filler is selected from the group consisting of carbon black, silica and mixtures thereof; and about 0.2 to about 5 parts of a curing agent, per 100 parts of said elastomer.
By employing a starch/emulsion styrene-butadiene copolymer as the reinforcing filler for vulcanizable elastomers, at least part of the conventional filler materials, carbon black and silica can be omitted. The novel use of these copolymer reinforcing fillers results in a more efficient dispersion thereof in the vulcanizable elastomers tan conventional fillers and the use of starch/plasticizer fillers and to provide tire treads for pneumatic tires having improved performance characteristics.